Charge pump generator and relative control method

ABSTRACT

The electric charge transferred in a charge transfer phase from the pump capacitor to the tank capacitor is diminished by reducing the amplitude of the voltage swing on the transfer capacitor proportionally to the current to be supplied. This is done by limiting the maximum voltage on the pump capacitor to a certain value. This maximum value is calculated to make the voltage on the transfer capacitor reach a certain minimum voltage at the end of the charge transfer phase. A charge pump generator includes a driving circuit that isolates the pump capacitor when the voltage on it reaches the maximum value.

FIELD OF THE INVENTION

The present invention relates to the field of charge pump generators andmore particularly to a method of controlling a charge pump generator anda related charge pump generator with reduced low-frequency noise.

BACKGROUND OF THE INVENTION

Charge pump voltage generators are largely used in many integratedcircuits (ICs) for supplying the ICs at a pre-established voltageV_(NEG) that should remain constant as the current absorbed by the loadvaries. An example of a common charge pump voltage generator is shown inFIG. 1. The output voltage V_(NEG) is regulated via a comparator C_(OMP)that compares it with a stable reference (control) voltage V_(REF) 1.

The circuit of FIG. 1 has operating phases in which the pump capacitorC_(P) is charged at a certain supply voltage VDD, alternated withoperating phases in which the pump capacitor C_(P) is coupled inanti-parallel manner to the charge tank capacitor C_(T), that suppliesthe electronic circuit with a voltage V_(NEG) of opposite sign inrespect to the charge voltage VDD. As long as the voltage V_(NEG) issmaller than the voltage V_(REF) 1, the pump capacitor C_(P) remainscoupled to the supply voltage VDD. When the voltage V_(NEG) exceeds thereference voltage V_(REF) 1 the capacitor C_(P) charges the tankcapacitor C_(T) when the clock signal CK assumes a logically activevalue, and is charged anew at the supply voltage VDD when the clocksignal CK becomes logically null.

In practice, this loop controls the duty cycle at a constant frequencywhen the charge current is above a certain threshold that depends uponthe supply voltage, the on-resistances R_(ON) of the switches SW1 andSW2, the pump capacitance C_(P) and the delay of the feedback line,constituted by the comparator and by the logic gates. T_(CK) being theperiod of the clock signal CK, and Q_(min) being the minimum chargetransferred from the pump capacitor C_(P) to the tank capacitor C_(T),the load I_(load) must absorb a minimum current I_(min) given by thefollowing equation:

$\begin{matrix}{I_{\min} = \frac{Q_{\min}}{T_{CK}}} & (1)\end{matrix}$to switch the switches SW1 and SW2 at each period of the clock signalCK.

If the current I_(load) is smaller than the value I_(min), the chargetransferred in a clock period from the capacitor C_(P) to the capacitorC_(T) is larger than that necessary for delivering this current for aclock period. The voltage V_(NEG) does not reach the threshold V_(REF)within the current period and the output of the AND gate remains nullfor more consecutive clock periods.

This situation is undesirable because it generates switching noise infrequency intervals that should be as free as possible from noise for acorrect operation of circuits supplied by the charge pump. Indeed, theswitches SW1 and SW2 generate switching noise centered around thefrequency of the clock signal, when they switch at each period of theclock signal CK, and at a smaller and smaller frequency if they do notswitch for more consecutive clock periods. This consequent low frequencynoise may disturb sensitive operation of circuits supplied by the chargepump.

The published patent application US 2002/0105312 to Texas InstrumentsInc. discloses a charge pump regulator with adjustable output current.In this device, the charging of the tank capacitor is regulated viaswitches with different on-resistances. A drawback of this approach isthat the switches with low on-resistance occupy a relatively largesilicon area.

SUMMARY OF THE INVENTION

This invention provides a method of controlling a charge pump generatorand a relative charge pump generator with a reduced low frequencyswitching noise and a reduced silicon area consumption.

According to the method of this invention, the electric chargetransferred in a charge transfer phase from the pump capacitor to thetank capacitor is diminished by reducing the amplitude of the voltageswing on the transfer capacitor proportionally to the current to besupplied. Preferably, this is done by limiting the maximum voltage onthe pump capacitor to a certain value. This maximum value is calculatedsuch to make the voltage on the transfer capacitor reach a certainminimum voltage at the end of the charge transfer phase.

The method of this invention is implemented in a charge pump generatorhaving a driving circuit that isolates the pump capacitor when thevoltage on it reaches the maximum value. In so doing, the abovementioned problems of low-frequency switching noise are overcome withoutrealizing switches of very low on-resistance, that require a relativelylarge silicon area.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram illustrating a charge pump generator inaccordance with the prior art.

FIG. 2 is a schematic diagram illustrating the control circuit of acharge pump generator in accordance with the present invention.

FIG. 3 is a schematic diagram illustrating a charge pump generator ofthe present invention.

FIG. 4 is a sample graph illustrating the operation of the charge pumpgenerator of FIG. 3.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

To center the noise generated by the switches SW1 and SW2 at thefrequency of the clock signal CK, the switches are switched at eachclock period. As a consequence, if the current I_(load) absorbed by theload supplied by the charge pump generator diminishes, it is necessaryto also diminish the value I_(min). This may be done only by reducingthe minimum charge transferred from the pump capacitor C_(P) to the tankcapacitor C_(T) proportionally to the current I_(load) absorbed by thecircuit supplied by the charge pump, because the period T_(CK) of theclock signal CK is generally fixed by design specifications.

Vcp_(START) being the voltage on the nodes of the pump capacitor at theinstant in which the switches SW1 and SW2 are turned on, T_(loop) beingthe duration of the time interval in which the switches SW1 and SW2 areturned on in a clock period,

$\begin{matrix}{{Q_{\min} \cong {\frac{{Vcp}_{START} - {V_{neg}}}{2 \cdot R_{ON}} \cdot T_{loop}}}{{and}\mspace{14mu}{thus}}} & (2) \\{I_{\min} = {\frac{Q_{\min}}{T_{CK}} \cong {\frac{{Vcp}_{START} - {V_{neg}}}{2 \cdot R_{ON}} \cdot \frac{T_{loop}}{T_{CK}}}}} & (3)\end{matrix}$R_(ON) being the on resistance of the two identical switches SW1 andSW2.

The voltage V_(NEG), as the period T_(CK) of the clock signal, is fixedby design specifications. The ratio

$\begin{matrix}\frac{T_{loop}}{T_{CK}} & (4)\end{matrix}$may hardly be modified with sufficient precision.

According to the method of this invention, the size of the two switchesSW1 and SW2 need not be increased to reduce their on-resistance, as perthe prior art approach. According to this invention, the voltageVcp_(START) is reduced such to make the current I_(min) equal to thecurrent I_(load) absorbed by the load. The charge Q_(min) transferredfrom the capacitor C_(P) to the capacitor C_(T) isQ _(min) =Q _(START) −Q _(END)  (5)Q_(START) and Q_(END) being the charge on the pump capacitor C_(P) atthe beginning and at the end of the charge transfer phase. By imposingthat the minimum current I_(min) be equal to the current absorbed by theload I_(load), the following equation holds:

$\begin{matrix}{I_{load} = \frac{Q_{START} - Q_{END}}{T_{CK}}} & (6)\end{matrix}$

Considering that the charge on the pump capacitor is proportional to thevoltage on it and that the proportionality factor is the capacitance,C_(P), the following equation may be written:

$\begin{matrix}{{Vcp}_{END} = {{Vcp}_{START} - \frac{I_{load} \cdot T_{CK}}{C_{P}}}} & (7)\end{matrix}$wherein Vcp_(END) is the voltage on the pump capacitor at the end of thetransfer charge phase. Equation (7), together with equation (3), allowsa determination of the values of the maximum and minimum voltage on thecapacitor C_(P) as a function of the current absorbed by the loadI_(load) and of the other parameters of the charge pump generator.

According to this invention, a control circuit for a charge pumpgenerator for establishing a maximum voltage Vcp_(START) on the pumpcapacitor C_(P) is depicted in FIG. 2. The circuit comprises a sensingamplifier (Sense) of the voltage on the pump capacitor that generates asignal V_(SNS) representing this voltage, including an operationalamplifier A₀, biased by a reference voltage V_(REF) 2, and the resistorsR₁ and R₂. The signal V_(SNS) is sampled by the circuit “Sample andHold” S/H when a sample and hold signal S/H is asserted, and a controllogic circuit P.I.CONTROL generates a signal corresponding to the valueVcp_(START) as a function of the difference V_(ERR) between the voltagesampled at the end of the charge phase Vcp_(END) and the desired valueVcp_(ENDtarget). Finally, a comparator COMPARATOR compares the voltageVcp currently on the pump capacitor with the value Vcp_(START), stoppingthe charging of the capacitor C_(P) via a logic signal STOP when thevoltage on it Vcp reaches the maximum desired voltage Vcp_(START).

When the logic signal STOP is asserted, the switches SW3 and SW4 areturned off and the charging phase of the pump capacitor C_(P) isstopped. By properly determining the voltage at the beginning of a newcharge phase Vcp_(START) according to equation (3), the voltage on thecapacitor C_(P) is exactly Vcp_(END) exactly when the regulated voltagesurpasses the reference threshold V_(REF) 1.

An embodiment of a charge pump generator of this invention, thatincludes the control circuit of FIG. 2, is shown in FIG. 3. The enablingsignal PUMP of the switches SW1 and SW2 is generated as shown in FIG. 1,while the enabling signal CHARGE of the switches SW3 and SW4 is thelogic NOR of the signal STOP and of the clock CK. In doing so, theenabling signal CHARGE of the switches that connect the pump capacitorC_(P) to the supply and to ground is disabled during a charging phase,started with a trailing edge of the clock CK as soon as the signal STOPbecomes logically active.

Results of simulations of the operation of the charge pump generator ofthe invention depicted in FIG. 3 are shown in FIG. 4. The value of thethreshold voltage V_(REF) 1 of FIG. 1 is set to −3V. When the clocksignal switches at each active logic value, a charge transfer phasestarts (schematically indicated in FIG. 4 with the label POMPA), thevoltage on the capacitor C_(P) diminishes because it transfers itscharge on the tank capacitor C_(T), and the absolute value of theregulated voltage V_(NEG) increases.

As soon as the regulated voltage becomes smaller than the thresholdV_(REF) 1, set to −3V in the shown example, the comparator COMP of FIG.1 switches and opens the switches SW1 and SW2. In this case theregulated voltage V_(NEG) is sustained only by the capacitor C_(T) andthus its absolute value decreases, while the voltage on the pumpcapacitor equals the voltage Vcp_(END).

When the clock signal CK switches low, a charge phase is started. Theregulated voltage is always sustained only by the tank capacitor C_(T)and thus its absolute value continues diminishing, while the switchesSW3 and SW4 are closed and the pump capacitor C_(P) charges. When thevoltage V_(CP) reaches the value Vcp_(START), the control circuit ofFIG. 2 of the charge pump generator of this invention opens the switchesSW3 and SW4 and stops charging the pump capacitor.

With the charge pump capacitor of this invention, the switching noisegenerated by the switches SW1 and SW2 remains substantially confinedaround the clock frequency, where it may be easily filtered withoutlimiting the performances of the circuits supplied by the charge pumpand without using purposely made low on-resistance switches, that occupya relatively large silicon area.

1. A method of controlling a charge pump generator having at least atank capacitor on which a regulated voltage is generated, and a pumpcapacitor coupled between a supply line and a ground node during chargephases and coupled in anti-parallel to the tank capacitor during chargetransfer phases, the method comprising: alternating charging phases ofthe pump capacitor with charge transfer phases; and reducing an electriccharge transferred from the pump capacitor to the tank capacitor duringa charge transfer phase proportionally to reductions of an outputcurrent, by reducing an amplitude of a voltage swing on the pumpcapacitor proportionally to the output current.
 2. The method of claim1, wherein the amplitude of the voltage swing is reduced by: comparing avoltage on the pump capacitor with a first voltage and comparing theregulated voltage with a reference voltage; and stopping charging phasesand charge transfer phases respectively to isolate the pump capacitorwhen the voltage thereon reaches the first voltage during a chargingphase and when the regulated voltage exceeds the reference voltageduring a charge transfer phase.
 3. The method of claim 2, wherein theamplitude of the voltage swing is reduced by reducing the first voltageproportionally to the output current.
 4. The method of claim 2, furthercomprising: beginning a charging phase when the regulated voltage on thetank capacitor surpasses the reference voltage and a clock signal of thecharge pump generator switches low; beginning a charge transfer phasewhen the regulated voltage exceeds the reference voltage and the clocksignal switches high; and setting the first voltage to make the voltageon the pump capacitor reach a second voltage at an end of the chargetransfer phase.
 5. A method of controlling a charge pump generatorcomprising a tank capacitor for outputting a regulated voltage, and apump capacitor coupled between a supply line and a ground node duringcharge phases and coupled in anti-parallel to the tank capacitor duringcharge transfer phases, the method comprising: alternating chargingphases of the pump capacitor with charge transfer phases; and reducing acharge transferred from the pump capacitor to the tank capacitor duringa charge transfer phase based upon reductions of a generator outputcurrent, by reducing a voltage swing on the pump capacitor based uponthe generator output current.
 6. The method of claim 5, wherein thevoltage swing is reduced by: comparing a voltage on the pump capacitorwith a first voltage and comparing the regulated voltage with areference voltage; and isolating the pump capacitor when the voltagethereon reaches the first voltage during a charging phase and when theregulated voltage exceeds the reference voltage during a charge transferphase.
 7. The method of claim 6, wherein the voltage swing is reduced byreducing the first voltage proportionally to the generator outputcurrent.
 8. The method of claim 6, further comprising: beginning acharging phase when the regulated voltage on the tank capacitor exceedsthe reference voltage and a clock signal of the charge pump generatorswitches low; beginning a charge transfer phase when the regulatedvoltage exceeds the reference voltage and the clock signal switcheshigh; and setting the first voltage to make the voltage on the pumpcapacitor reach a second voltage at an end of the charge transfer phase.9. A charge pump generator comprising: a tank capacitor to output aregulated voltage; a pump capacitor coupled between a supply line and aground node during charging phases and coupled in anti-parallel to saidtank capacitor during charge transfer phases alternatingly with thecharging phases; switches connecting said pump capacitor between thesupply line and the ground node, and connecting said pump capacitor tothe tank capacitor; and a control circuit generating control signals forsaid switches to connect said pump capacitor to the supply line and tothe ground node during the charging phases, and to connect said pumpcapacitor in anti-parallel to the tank capacitor during the chargetransfer phases; said control circuit switching said switches to reducea voltage swing on the pump capacitor based upon a generator outputcurrent.
 10. The charge pump generator of claim 9, wherein said controlcircuit comprises: a logic comparison circuit for comparing a voltage onthe pump capacitor with a first voltage and for comparing the regulatedvoltage with a reference voltage, for generating logic control signalsfor said switches to isolate the pump capacitor when the voltage thereonreaches the first voltage during the charging phases and when theregulated voltage surpasses the reference voltage during a chargetransfer phase, respectively.
 11. The charge pump generator of claim 10,wherein said logic comparison circuit comprises: a comparator to comparethe regulated voltage with the reference voltage to generate a logicallyactive comparison signal when the regulated voltage exceeds thereference voltage; a logic circuit generating a first control signal fora first pair of said switches connecting the pump capacitor inanti-parallel to the tank capacitor based upon the comparison signal anda generator clock signal, and generating a second control signal for asecond pair of said switches connecting the pump capacitor between thesupply line and the ground node as an inversion of the first controlsignal; a detection circuit for detecting the voltage on the pumpcapacitor, and generating a detection signal based thereon; asample/hold circuit being input with the detection signal, generatingthe first voltage as a function of a second voltage and of the voltageon the pump capacitor at the end of the charge transfer phase; a secondcomparator generating a second logic comparison signal when the voltagesensed on the pump capacitor reaches the first voltage; and the logiccircuit including a logic NOR gate input with the second logiccomparison signal and the clock signal, and generating the secondcontrol signal for the second pair of switches.